The current approach to protection of signals transmitted over optical communications links is to perform an analog-to-digital conversion, apply digital encryption to the digital data stream, and convert the digital data stream back to analog. For example, U.S. Pat. No. 5,864,625 (“Methods and Apparatus for Secure Optical Communications Links”) teaches a method whereby communications information is encrypted with a security key. An optical beam is then modulated with both the security key and the encrypted communications information, with different modulation schemes being used for each. The dual-modulated optical beam is then transmitted to a receiver. At the receiver, the optical beam is split into first and second optical beams. First and second demodulators are then employed to demodulate the optical beams to recover the encrypted communications information and its data rates, and the security key and its data rate. The encrypted communications information, the security key, and the data rate information are then sent to decryption and timing circuitry that decrypts the encrypted communications information to obtain the original unencrypted communications information.
Likewise, U.S. Pat. No. 5,793,871 (“Optical Encryption Interface”) discloses an analog optical encryption system based on phase scrambling of two-dimensional optical images and holographic transformation for achieving large encryption keys and high encryption speed. An enciphering interface uses a spatial light modulator for converting a digital data stream into a two-dimensional optical image. The optical image is further transformed into a hologram with a random phase distribution. The hologram is converted into digital form for transmission over a shared information channel. A respective deciphering interface at a receiver reverses the encrypting process by using a phase conjugate reconstruction of the phase scrambled hologram.
However, the above listed examples both require inline digital conversion and encryption. These additional steps increase the complexity of the system and interfere with optimal use of the communications link. The analog conversion and data encryption steps introduce a severe bottleneck in an optical communications path due to the limited performance of digital electronics when compared to the extremely high bandwidth of optical fiber. It would be advantageous then to eliminate the inline digital conversion and encryption bottleneck so that unconverted analog signals can be transmitted in their native mode in a timely manner without adding additional hardware requirements for encryption/decryption.